Also manually:
1- Edit any external functions to return an object. I personally used user/externallib.php:create_users(). i edited it to return user objects with id/username instead of arrays with id/username.
2- run the PHP-SOAP client (https://github.com/moodlehq/sample-ws-clients/tree/master/PHP-SOAP). You can test with REST too but you'll need to cheery-pick MDL-37358.

phpunit externallib_testcase lib/tests/externallib_test.php
Also manually:
1- Edit any external functions to return an object. I personally used user/externallib.php:create_users(). i edited it to return user objects with id/username instead of arrays with id/username.
2- run the PHP-SOAP client ( https://github.com/moodlehq/sample-ws-clients/tree/master/PHP-SOAP ). You can test with REST too but you'll need to cheery-pick MDL-37358 .
You should see the resulting XML code with the new user id/username.